1. Field of the Invention
This invention relates to certain covering power, heat developable photographic materials and processes for developing a positive image with the described materials. In one of its aspects it relates to a covering power, heat developable photographic element comprising the described combination of components capable of providing a positive developed image. In another aspect it relates to a covering power, heat developable photographic composition comprising the described combination. A further aspect of the invention relates to a process of developing a positive image in a covering power, heat developable photographic material containing the described imaging combination.
2. Description of the State of the Art
It is known to obtain an image in an imaging material, especially a photographic imaging material, by what is known as dry processing with heat. These materials are sometimes described as heat developable photographic materials or photothermographic materials. Such heat developable photographic materials are imagewise exposed to provide a latent image. They are then heated to provide a developed image in the absence of processing solutions or baths. An example of a heat developable photographic material which comprises a photosensitive silver halide with an image-forming combination comprising (1) a silver salt of a certain 1,2,4-mercaptotriazole derivative with a silver halide developing agent is described, for example, in Research Disclosure, June 1977, Volume 158, Item No. 15869, of P. D. Knight et al, published by Industrial Opportunities Limited, Homewell, Havant Hampshire, P09 1EF, UK. Another example of a heat developable photographic material is described in U.S. Pat. No. 3,785,830 of Sullivan et al, issued Jan. 15, 1974. This heat developable photographic material contains photosensitive silver halide with an image-forming combination of an organic silver salt oxidizing agent containing a certain heterocyclic thione ligand and a reducing agent. These heat developable photographic materials are useful for providing a developed image in the absence of processing solutions or baths; but, the materials have not been suggested for providing a developed image with covering power, reversal imaging.
Photographic materials are known which can provide development of an image in such materials by what is known as covering power imaging. A fundamental difference exists between conventional silver development processes and what are described as covering power imaging processes. In most conventional photographic silver materials, changes in optical density with exposure arise primarily from differences in the amount of silver reduced at the latent image sites. In silver covering power imaging the amount of silver reduced does not vary greatly with exposure. In such covering power imaging, density variations at image areas are due, for the most part, to differences in silver covering power at various exposure levels. Silver covering power imaging, as used herein, is intended to mean imaging in a photographic silver material in which variations in optical density of the developed image derive primarily from variations in silver covering power as a function of exposure. Covering power, as used herein, is intended to mean the measure of the silver opacity in the developed image and is derived arithmetically by dividing (a) the optical density by (b) the grams of developed silver per unit area, such as per square decimeter, in the layer of the material containing the developed image. Covering power and covering power imaging are known in conventional photographic materials which provide development with processing solutions or baths. This is described, for example, in the article by K. Murofushi, J. Soc. Sci. Phot., Japan, Volume 30 (4), 193-198 (1967), Canadian Pat. No. 808,585, and "The Theory of the Photographic Process", 3rd Edition, by Mees, 1966, pages 74-75 and 419-420. Covering power imaging has also been described in heat developable photographic silver elements, such as in Research Disclosure, Volume 151, November 1976, Item No. 15168, of P. B. Merkel. None of the photographic materials described have used covering power in heat developable photographic materials with certain nucleating agents to enable reversal imaging.
Photographic silver halide materials are known for producing reversal or direct-positive images with processing solutions or baths. None of these photographic materials have been used for producing covering power, reversal imaging with a heat developable photographic element.
It has been desirable to provide a heat developable photographic material and process which enable development of a reversal or positive image with a reduced concentration of silver and in the absence of processing solutions or baths. The heat developable photographic materials described have not enabled these advantages and also have not enabled the desired degree of reduction of silver in a heat developable material with the production of a positive or reversal image.